Anderson Dam Seismic Retrofit Drilling Program Phase 9
Summary
SCH Number
2024070547
Public Agency
Santa Clara Valley Water District
Document Title
Anderson Dam Seismic Retrofit Drilling Program Phase 9
Document Type
NOE - Notice of Exemption
Received
Posted
7/16/2024
Document Description
Additional geotechnical investigations are required to characterize seismic risk, foundation conditions, and dam materials to support the planning and design processes for the proposed Anderson Dam Seismic Retrofit Project (ADSRP). The investigation would include up to 10 borings and excavation of 7 fault trenches, followed by laboratory testing of the collected samples. Two borings (BB-03 and BB-04) would be drilled a minimum of 40 feet below ground surface (bgs) adjacent to the north channel of Coyote Creek to support the design of a future bridge. At the dam crest near the right abutment, four soil and rock core borings (BU-44 through BU-47) would be drilled up to approximately 230 feet bgs along an existing access road to investigate subsurface conditions at and near the contact of the dam embankment and dam foundation. Up to two soil borings (RW-01 and RW-02) are planned to support the design of a new 210-foot long, up to 16-foot-tall retaining wall by the proposed ADSRP site entrance. The boring(s) would be drilled up to approximately 75 feet bgs. At the low-level outlet structure, two planned borings (OW-40 and OW-41) would be drilled approximately 80-100 feet bgs to evaluate the potential for faulting in the low-level outlet structure foundation.
All proposed borings, except the BU borings on the dam crest, would be drilled with a track-mounted drill rig. Borings to be drilled along the dam crest would be drilled with a truck-mounted drill rig. All drill cuttings would be contained during the drilling process and would be placed into drums or bins and disposed of at an off-site landfill upon completion of drilling.
After drilling is complete, three open standpipe piezometers would be installed within the bedrock foundation in borings BU-44, BU-45, and BU-47, to determine seepage potential below the dam embankment near the right abutment. Two temporary open standpipe piezometers would be installed in boreholes OW-40 and OW-41 to inform the dewatering design of trenches T10N and T10S, as further described below. One vibrating wire piezometer would be installed within the upper foundation in borings BU-44, BU-45, and BU-47, and two vibrating wire piezometers would be installed at the base of the embankment, and one within the foundation in boring BU-46, to determine the depth to groundwater in the embankment and the influence of seepage near the right abutment. Following completion of all drilling activities, borings would either be backfilled with neat cement grout or bentonite-cement grout.
Adjacent to or near the low-level outlet structure, high level outlet works shaft, tunnel portal, and spillway, seven trenches ranging from approximately 2 to 10 feet deep, and approximately 35 to 250 feet long would be excavated to investigate foundation materials and to evaluate faulting properties and age. To excavate two trenches (T10-N and T10-S), localized groundwater dewatering would be required; pumped groundwater would be discharged to Coyote Creek. Dewatering wells would be installed using a bucket auger. Once the wells are installed, pumps would be added to the wells and associated piping would be installed, and groundwater dewatering would commence. Dewatering is expected to take approximately one to four weeks to complete, depending on the permeability of the subsurface materials. All water would be directly pumped into Coyote Creek, and erosion protection would be added at the discharge point to minimize or prevent erosion. Groundwater quality in the Santa Clara Subbasin is generally of very good quality, and temporary discharge of the small volumes of groundwater from the two trenches into Coyote Creek is unlikely to result in any exceedances of water quality standards as defined by the San Francisco Bay Regional Water Quality Control Board. Regardless, water quality would be monitored upstream and downstream of the discharge point to ensure that water quality is maintained and not degraded throughout project activities. Excavation operations for trenching at these two locations would commence once dewatering has lowered the groundwater level to below the maximum excavation depth. Upon completion of excavation activities, each trench would be excavated and backfilled separately, and ground conditions would be restored to pre-project conditions to the extent feasible.
Contact Information
Name
Abby Annicchiarico
Agency Name
Santa Clara Valley Water District
Job Title
Associate Environmental Planner
Contact Types
Lead/Public Agency / Project Applicant
Phone
Location
Cities
Morgan Hill
Counties
Santa Clara
Regions
Countywide
Waterways
Anderson Reservoir, Coyote Creek
Notice of Exemption
Exempt Status
Categorical Exemption
Type, Section or Code
§15306, Information Collection
Reasons for Exemption
The CEQA Guidelines §15306 exemption “consists of basic data collection, research, experimental management, and resource evaluation activities which do not result in a serious or major disturbance to an environmental resource. These may be strictly for information gathering purposes, or as part of a study leading to an action which a public agency has not yet approved, adopted, or funded.” The proposed project would include up to 10 geotechnical borings and excavation of 7 fault trenches, followed by laboratory testing of the collected samples. The CEQA Guidelines §15306 exemption applies because the proposed project consists of basic data collection activities for information-gathering purposes which, as demonstrated by the below project description, would not result in a serious or major disturbance to an environmental resource.
Furthermore, none of the exceptions to the use of a Categorical Exemption noted under CEQA Guidelines §15300.2 would apply. The proposed project would not: impact an officially adopted environmental resource of hazardous or critical concern, result in a significant cumulative impact, result in a significant impact due to unusual circumstances, damage scenic resources, be located in a listed hazardous waste site, or cause a significant impact on historical resources.
County Clerk
Santa Clara
Attachments
Notice of Exemption
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